Agile Smart-Sensor Design with Scalable Wireless MCUs
Contributed By Digi-Key's European Editors
Wireless connectivity is a key aspect of intelligent connected devices as diverse as smart lightbulbs, remote controls, smart-building sensors and actuators, smart meters, wearables including wellness devices, security alarms, and beacons.
Selecting the right wireless technology is not always straightforward, as multiple standards may be applicable, and markets and end-user preferences continue to evolve. Development teams have to begin work well in advance to meet the intended launch date, but committing to a wireless technology at such an early stage may not be ideal. On the other hand, some types of products may benefit from the flexibility to offer variants with different radio options for sale in different territories or markets.
Nevertheless, the radio is a core component of an intelligent sensing application, and traditional approaches demand an early decision about which standard to adopt. Consider a manufacturer who has chosen ZigBee® technology for an application. In practice, this decision locks in many aspects of the design such as the transceiver technology, PCB layout, software stack, and the API to access the radio.
Suppose new market data emerges as the design is progressing, which suggests that using Bluetooth® Smart would substantially increase the target market for the end product. To change the radio, the application will need to be adapted to the Bluetooth Smart stack and new API, and much of the work around the existing radio design would need to be scrapped. In effect, the challenge is to begin an almost complete redesign at a late stage in the project. Effectively, the team must choose between meeting the intended market window but with the wrong product, or launching the right product too late. Added engineering costs need to be considered as well.
Embedded platform for scalable wireless
To give manufacturers greater flexibility in their choice of wireless technology, Texas Instruments (TI) has created the SimpleLink™ ultra-low-power wireless microcontroller (MCU) platform. The architecture is based on the ARM® Cortex®-M3 and currently offers memory configurations from 32 KB up to 128 KB Flash. It provides enough processing capacity to serve as a standalone MCU for a wide range of intelligent sensing applications.
SimpleLink has been designed to enable scalability across wireless technologies. The family of devices in pin-to-pin compatible package options supports a range of different radios, including Bluetooth Smart, Sub-1 GHz, ZigBee, 6LoWPAN, IEEE 802.15.4, RF4CE™ and proprietary modes operating up to 5 Mbps.
From a hardware standpoint, it is straightforward to change to a device with a different integrated radio. All 2.4 GHz technologies and all Sub-1 GHz technologies are directly pin-to-pin compatible. In addition, all of the other peripherals are the same between SimpleLink devices. This gives manufacturers a great deal of flexibility in being able to delay final selection of the radio technology until late in the design process.
The platform is also code compatible across each of the different standards it supports. Switching radios, however, does have some impact on application software design. This arises from the differences in the radio stacks, which must be accounted for by the application. For example, interfacing to the 6LoWPAN stack is done using IP messages. With Bluetooth Smart, the application reads or modifies various attributes. These differences are captured in the APIs TI supplies with each of its SimpleLink wireless MCUs.
As a best practice, manufacturers can design the radio interface in a modular fashion. Rather than have the application directly access the radio, the wireless API can be abstracted by having the application send data to a radio function. This function can then process data to be transmitted or received as required using the appropriate API. The effect is that, to change radios late in the design process, only this radio function would require porting.
Same devices, different radios
The platform comprises the CC2640 wireless MCU for Bluetooth Smart, the CC2630 supporting 6LoWPAN and ZigBee, CC1310 for Sub-1 GHz, and the CC2620 supporting ZigBee RF4CE. The devices are available in multiple package styles as shown in Figure 1.
Figure 1: The SimpleLink strategy eliminates hardware issues from the wireless-reassignment challenges.
TI is also announcing the CC2650 multi-standard device. This “superset” device can be dynamically configured in both hardware and software to support one of several different 2.4 GHz radios. Designs built with the CC2650 can go to production without locking in a selection and be configured at the time of installation in the field. This allows manufacturers to truly wait until the last minute to decide on which radio to implement without changing the antenna design.
The CC2650 also enables applications to support multiple radios with a single chip by allowing the supported radio to be changed. Thus, by reprogramming the CC2650 in the field, a system could communicate with both ZigBee- and Bluetooth-based devices.
The SimpleLink platform integrates multiple processors to provide the different levels of computational capabilities required for the variety of tasks an intelligent-sensing application performs, as Figure 2 shows. Using the right processor for the task at hand enables the device to operate at the lowest possible power.
Figure 2: SimpleLink architecture minimizes the energy consumed to sense, process and communicate.
The Application Processor is an ARM® Cortex®-M3, which serves as the main processor of the SimpleLink ultra-low-power platform. It provides the performance needed to serve as a standalone MCU that can intelligently manage a sensor-based system. The Cortex-M3 provides plenty of processing power to handle the application and high-level stack processing, and is extremely energy efficient.
The Radio Processor is an integrated Cortex-M0, which is dedicated to managing all low-level radio tasks for the system. This offloads timing-critical tasks from the main CPU.
The third processor is an ultra-low-power integrated MCU dedicated to fast and efficient sensor monitoring. This Sensor Controller is designed to provide exactly the right level of processing required for sampling data and making simple sensor decisions. In addition, it has limited memory and no extraneous peripherals. It is extremely power efficient for tasks such as regularly polling a sensor output and determining if a threshold event has occurred, and avoids having to wastefully wake up the main CPU when this is not needed.
TI has simplified design using SimpleLink wireless MCUs by providing the software needed to operate and interface to the wireless radio. This simplifies radio design to the degree that developers can drop in the appropriate SimpleLink device and quickly begin using the radio without a lot of configuration or tuning. To this end, the radio controller is provided with production code that has been optimized to achieve the most efficient radio operation.
Because the Sensor Controller needs to monitor sensors, make decisions, and take action based on the particular application, developers need to be able to configure its operation. TI provides a software development tool, Sensor Controller Studio, which allows users to configure the sensor controller. This tool outputs a Sensor Controller Interface driver, which incorporates the generated Sensor Controller machine code and associated definitions. It is possible to configure the sensor controller to perform common tasks without having to write any code, while for applications that require custom code, this is supported through a C-like scripting language. Sensor Controller Studio speeds up development by using the sensor controller for testing and debugging functionality. This allows for live visualization of sensor data and algorithm verification.
Another key advantage of the Sensor Controller is that it is integrated with the main CPU. Traditionally, any additional sensor controller would be implemented using a second, less powerful MCU to offload the main application processor. This can deliver power savings by allowing the application processor to remain in sleep mode while the lower-power sensor controller monitors and manages the sensors. On the other hand, because the secondary MCU is external to the application processor, developers would have to design and manage communication between the processors, and also implement interrupt capabilities to enable the controller to wake the application processor.
In the SimpleLink platform, the way the sensor controller is implemented provides all of the advantages of power efficiency without the disadvantage of complicating design. Because the sensor controller, radio MCU, and application processor are integrated on the same silicon, this greatly simplifies hardware and software design.
The SimpleLink platform provides a wireless MCU that is easy to program and avoids the challenges associated with trying to integrate PHYs and stacks. Application code runs on the ARM Cortex-M3, a standard MCU that many designers are already familiar with. TI provides an API for each of its radio technologies, thereby minimizing the learning curve for developers. RF and antenna design have been simplified as well without compromising reliability or performance. Robust security is built-in, and the protocol stacks are ready for production.
Designing with SimpleLink
To design with SimpleLink devices, developers can choose from full-featured design environments like the Code Composer Studio™ Integrated Development Environment (IDE) or IAR Embedded Workbench.
Evaluation kits are available that can be used to jumpstart design. Among these, the SimpleLink CC2650 Development Kit includes two CC2650 evaluation modules and two SmartRF06 motherboards that are designed to support software development and running radio performance tests. Because the CC2650 is capable of supporting multiple 2.4 GHz radio standards, this platform can also be used in application development for the CC2640 Bluetooth Smart and CC2630 ZigBee®/6LoWPAN wireless MCUs. The microcontrollers in the kit are pre-programmed with software for range testing. Bluetooth Low Energy and ZigBee stacks built on top of TI-RTOS, which integrates device drivers and power management, are also included.
The CC2650 wireless MCU is also at the heart of TI’s SensorTag IoT Kit. The SensorTag is ready to connect to the Cloud, with no programming experience needed to get started. It contains ten sensors including light, humidity and pressure sensors, a digital microphone, magnetic sensor, accelerometer, gyroscope, magnetometer, object-temperature sensor, and ambient-temperature sensor, and built-in iBeacon technology. The associated mobile app allows the user to see the sensor readings instantaneously upon startup, and customize content based on SensorTag data and physical location.
TI’s SimpleLink ultra-low-power wireless MCU platform simplifies the development of smart wireless devices and gives development teams the flexibility to change to a different wireless standard if necessary, even at a late stage of the design cycle. This allows projects to begin sooner – and final decisions to be made later – enabling OEMs to deliver optimized products to market at the right time. The energy-conscious multi-processor architecture helps developers more easily meet the strict power and performance requirements of many intelligent-sensing applications.
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